1. Field of the Invention
This invention relates to an image pickup apparatus for photoelectrically transferring optical energy of an image to electric signals as image data and recording the image data into a recording medium after processing predetermined image processing.
2. Description of the Related Art
This type of equipment with image pickup apparatus is described below by taking a digital camera as an example.
A digital camera (hereinafter abbreviated as camera) generally has no mechanical shutter in order to downsize and to simplify the camera. Exposure of the camera is controlled by not only varying exposure time (charge storage time) of an image pickup device such as CCD (Charge Coupled Device), but also varying a size or an area of an aperture mechanism. The camera calculates a proper exposure time t and the size of the aperture by a predetermined program corresponding to a luminance of an object.
On the other hand, in the camera using the image pickup device, it is impossible to control the exposure control time txe2x80x2 as an optional value, since the exposure time of the image pickup device is governed by a scanning time tr0 for one horizontal line of the image pickup device. Thus, the camera uses the scanning time tr0 for one horizontal line of the image pickup device as a standard time unit for calculating an exposure control time txe2x80x2. Hereupon, tr0=1/(30xc3x97512)≈65 xcexcs. The camera calculates the exposure control time txe2x80x2 by integral multiply of the standard time unit tr0.
When an exposure value EV corresponding to the luminance of the object is 15 (EV=15) and an aperture value AV corresponding to the size of the aperture is 4 (AV=4), a time value TV corresponding to the proper exposure time t becomes 11 (TV=EVxe2x88x92AV=15xe2x88x924=11). The proper exposure time t corresponding to the time value TV=11 becomes 1/2048 sec.
On the other hand, the camera selects a time of 7/(30xc3x97512)≈1/2194 sec which is the nearest time to the proper exposure time t as the exposure control time txe2x80x2 of the image pickup device. When the exposure control time txe2x80x2=1/2194 sec is converted to an exposure control time value TVxe2x80x2 of an APEX time value, the exposure control time value TVxe2x80x2 becomes 11.1 (TVxe2x80x2=11.1). Therefor, an error component xcex94TVe=TVxe2x80x2xe2x88x92TV=0.1 occurs.
When the error component xcex94TVe is smaller than an exposure control step xcex94EV which is, for example, ⅕(=0.2), there is no problem in the density of an image displayed on a monitor display. Thus, the photoelectric transferring operation of the image pickup device is controlled by using the exposure control time txe2x80x2 or the exposure control time value TVxe2x80x2.
In view of further downsizing and simplifying the digital camera, it is proposed to use the monitor display such as LCD (Liquid Crystal Display) as a viewfinder. In such a camera, it is proposed to fix the size of the aperture immovably and to control the exposure by only varying the exposure control time txe2x80x2 of the image pickup device when an image is taken for smoothly displaying on the monitor display serving as the viewfinder.
The camera, however, can select the exposure control time txe2x80x2 calculated by multiplying an integer on the standard time unit tr0. Thus, when the exposure is controlled by varying the exposure control time txe2x80x2 of the image pickup device without varying the size of thee aperture, the exposure cannot be controlled properly, especially in a highlight scene in which the exposure control time txe2x80x2 calculated by the luminance of the object becomes shorter than the standard time unit tr0.
Alternatively, even when a plurality of the exposure control time txe2x80x2 can be selected by using another time unit tr shorter than the standard time unit tr0 for realizing a delicate exposure control, an error component xcex94t between the proper exposure time t and the exposure control time txe2x80x2 becomes larger than the time unit tr in a highlight scene, so that the exposure cannot be controlled properly in the highlight scene.
Furthermore, it is assumed that the exposure control time txe2x80x2 can be variable by 1/8 TV. When the exposure value EV is equal to or larger than 15 (EV=15) in the above-mentioned example, the error component xcex94TVe between the proper exposure time value TV and the exposure control time value TVxe2x80x2 becomes larger than 1/8. Thus, the exposure control time value TVxe2x80x2 cannot be varied substantially. For example, with respect to EV=15.625 to 15.875 in a table 1 shown below, values of the proper exposure time value TV correspond to 11.625 to 11.875. However, the exposure control time value TVxe2x80x2 which can be selected and smaller than the proper exposure time value TV is 11.907 (txe2x80x2=4/(30xc3x97512) sec). Thus, the exposure control time value TVxe2x80x2 of the image pickup device is not varied in a region of the exposure value EV=15.625 to 15.875.
Furthermore, when the time value TV=14 is converted to an actual exposure time, it becomes nearly equal to 0.94/(30xc3x97512) sec which is shorter than the standard time unit tr0=1/(30xc3x97512) sec. Thus, the exposure control time txe2x80x2, which can be selected in the highlight scene equal to or higher than TV=14, becomes a constant value of 1/(30xc3x97512) sec, so that all images in the highlight scene will be over-exposed.
An object of this invention is to provide an image pickup apparatus by which an image with a proper density of colors can be taken even when an aperture is immovable and only an exposure time of an image pickup device is controlled.
An image pickup apparatus in accordance with this invention comprises an image sensor for photoelectrically converting optical energy of an image of an object to electric signals; a detector for sensing a luminance of the object; a first setter for setting a first exposure time which is calculated by integral multiply of a first time unit corresponding to the luminance sensed by the detector; a second setter for setting a second exposure time which is calculated by integral multiply of a second time unit shorter than the first time unit corresponding to the luminance sensed by the detector; a calculator for calculating a value of signal amplification due to a difference between the first exposure time and the second exposure time; and an amplifier for amplifying image signals obtained by exposing the image sensor by the first exposure time based on the value of the signal amplification calculated by the calculator.
By the above-mentioned configuration, an image having a proper density can be obtained without using adjustable aperture mechanism and shutter mechanism.